Polar decomposition consists of representing an arbitrary Mueller matrix with a product of three simpler matrices, but, since these matrices do not commute, the result depends on the order in which they are multiplied. We show that the six possible decompositions can be classified into two families and that one of these families always leads to physical elementary matrices, whereas the other does not.

We propose to analyze Shannon entropy properties of partially coherent and partially polarized light with Gaussian probability distributions. It is shown that the Shannon entropy is a sum of simple functions of the intensity, of the degrees of polarization, and of the intrinsic degrees of coherence that have been recently introduced. This analysis clearly demonstrates the contribution of partial polarization and of partial coherence to the characterization of disorder of the light provided by the Shannon entropy, which is a standard measure of randomness. We illustrate these results on two simple examples.